In the fabrication of semiconductor devices, various materials may be deposited on a substrate for various purposes. For example, inter-layer dielectric (ILD) material may be deposited and patterned with trenches. Metal material may then be placed in the trenches to form interconnect lines, or “traces”.
A barrier layer is often first formed within the trenches in an attempt to prevent diffusion of metal ions into the ILD material or the substrate during device operation. For example, a barrier layer of tantalum may first be deposited on the bottom and sides of a trench before filling in the remainder of the trench with a conductive metal material such as copper. This barrier layer may prevent ions of the conductive metal from diffusing beyond the trenches when current flows in the metal line during operation of the circuit. This diffusion, often referred to as electromigration, may eventually lead to the formation of voids in the metal lines, which may cause degraded operation or even failure of the device. Thus, the initial deposition of a barrier layer may be used to reduce electromigration over the expected operational life of the integrated circuit.
Unfortunately, a barrier layer formed as indicated above does not completely prevent diffusion or migration of all metal ions. For example, where copper is the metal deposited, even tantalum, a fairly good barrier for copper, is not able to prevent a significant amount of migration of copper ions out of the trenches. Copper is highly desirable for metal lines because it has high electrical conductivity and is easy to handle in the manufacturing operations. But it is also highly diffusive and often difficult to entirely isolate. As a result, the likelihood of electromigration-induced voids forming in copper metal lines formed in the convention manner is undesirably high.